Today, more than ever, wireless network operators are looking for ways to increase the efficiency, traffic capability and robustness of their radio communication systems. A well-known way of providing efficiency, in terms of capacity per Hertz of bandwidth used, is to employ co-channel dual polarization (CCDP) operation in the radio link. CCDP operation provides two parallel communication channels with orthogonal polarized signals (i.e. one vertically polarized signal and one horizontally polarized signal) over the same channel, and thus doubling the capacity of the radio link. However, despite the orthogonality of the signals some interference between them, also known as cross polarization interference, almost inevitably occurs, due to for example imperfect antenna isolation and channel degradation. The cross polarization interference may seriously degrade the performance of the radio link.
A well-known technique to mitigate the effects of the cross polarization interference between the signals is to use a cross polarization interference cancelation (XPIC) system, wherein two separate receiver modems, one for each polarization, processes and combines the signals coming from the two receiving paths to cancel-out the cross polar interference and recover the original independent signals.
In addition to the cross polarization interference between signals a radio link may suffer from other degrading factors such as high transmission loss due to, for instance, multipath fading or flat fading that could deteriorate the quality of the channel. Automatic transmit power control (ATPC) is a common technique utilized in microwave radio links to control the transmitter power in relationship to the strength of the received signal to mitigate transmission losses in the radio link. In general, the ATPC performs an adjustment of the transmit radio power in order to maintain constant receive signal level by detecting a drop in the received signal level and transmit information about the drop to the transmit side, via a radio service channel, to provide for an adjustment of the transmit power. In this way the reliability may be increased in term of minimum time of outage of the radio link due to loss of signal.
Thus, by implementing an ATPC control loop in a radio link employing XPIC will allow for an increase in both capability of the radio link as well as an increase of the reliability. However, assume that the two independently ATPC loops are working and that one of the two orthogonality polarized signals, for instance the vertical polarized signal, is affected by fading. This will result in a drop in the vertically polarized signal and, consequently, the related ATPC loop performs an increase of the transmit power only on the vertical antenna. Since the transmit power of the horizontal polarized signal remains constant, then a rise of the transmit power of the vertical polarized signal will result in an increasing in interference, i.e. the cross polarization interference (XPI), from the vertical polarized signal on horizontal polarized signal degrading the performance of the radio link. Thus the radio link is in this way not robust against asymmetrical fading.
Although limited to counteract only Rayleigh fading, NEC Corp. (in their U.S. Pat. No. 5,392,459) gives a partial solution to the problem of handling asymmetrical fading. The concept adopted by NEC Corp. is to use a single ATPC control for both polarized signals instead of two independent for the two orthogonal channels. The transmit powers of the polarized signals are simultaneously increased when a drop of the receive signal level of at least one of the two polarized signals is detected. However, the equal increase of both the horizontally and the vertically polarized signals for preventing cross polar interference due to the selective fading, generates as a consequence an unnecessary overpower of the channels not affected by the fading, which may degrade the radio link.
Thus, finding a way to realize an ATPC loop in an XPIC microwave radio link which allows for an increased tolerance against cross polarization interference and at the same time is capable of handling imbalances between the two polarized signals is therefore highly sought after.